Two families of exocomets in the β Pictoris system



The young planetary system surrounding the star β Pictoris harbours active minor bodies1,2,3,4,5,6. These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of our Solar System7. Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies8,9,10,11, that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, which may be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.

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Figure 1: A typical Ca ii spectrum of β Pictoris.
Figure 2: Coma absorption depths as a function of surface ratio for transiting exocomets.
Figure 3: Measured distribution of the physical properties of transiting exocomets.
Figure 4: Periastron distance versus periastron longitude.


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This work was based on observations made with ESO telescopes at the La Silla Observatory and data obtained from the ESO Science Archive Facility. This work has been supported by an award from the Fondation Simone et Cino Del Duca. We also acknowledge support from the French Agence Nationale de la Recherche (ANR), under programmes ANR-12-BS05-0012 Exo-Atmos and ANR-2010 BLAN-0505-01 (EXOZODI). We thank P. A. Wilson for his comments on the manuscript.

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F.K. led the data analysis, with contributions from A.L.d.E., J.B., A.V.-M., R.F. and G.H.; F.K. and A.L.d.E. wrote the paper; H.B. computed mean-motion resonance curves and developed theoretical modelling; A.L.d.E., A.V.-M., A.-M.L. and H.B. contributed to the conception of the project. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to F. Kiefer or A. Lecavelier des Etangs.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The Na i spectrum of β Pictoris.

a, Na i D2-line spectrum (5,890 Å). b, Na i D1-line spectrum (5,896 Å). It shows the superposition of all Na i spectra of β Pic (black dots) compared with the stellar reference spectrum (red line). Radial velocities are given in the star’s rest frame. The stable Na i line centred at the star’s radial velocity is identified as due to the circumstellar (CS) disk. The sharpness of the Na i D1 and D2 lines and the steadiness of this circumstellar feature in all spectra confirm the stability of HARPS on a timescale of years. The narrow absorption lines seen in most of the spectra and not in the calculated reference spectrum are due to atmospheric water.

Extended Data Figure 2 The Ca ii reference spectrum of β Pictoris.

a, Ca ii K-line spectrum (3,933.66 Å). b, Ca ii H-line spectrum (3,968.47 Å). It shows the superposition of all the Ca ii spectra of β Pic (black dots) compared with the stellar reference spectrum (red line). The stable circumstellar (CS) line is centred at the star’s radial velocity. Variable absorption features are revealed by their diffuse shapes with respect to the dark upper envelop of the cloud of points. The predominance of redshifted absorption features is clearly visible. A small interstellar (IS) line is noticeable on the left of the circumstellar line24.

Extended Data Figure 3 A typical fitted Ca ii normalized spectrum.

a, Ca ii K line normalized spectrum. b, Ca ii H line normalized spectrum. The Ca ii normalized spectrum (black line) is obtained through the division of the corresponding regular spectrum collected on the 27 October 2009 (Fig. 1) by the reference spectrum plotted in Extended Data Fig. 2. Radial velocities are given with respect to the stellar rest frame. The fit of each feature detected is detailed with red dashed lines, and their superposition with a solid red line. The bottom panels show the residuals of the fit. The grey shading indicates the ±3 km s−1 excluded CS region, where variable absorption features caused by exocomets are not resolved from the circumstellar line. This spectrum presents all types of variable absorption features: a broad and shallow absorption at large radial velocity (±50 km s−1) and a sharp and deep absorption at small radial velocity (20 km s−1).

Extended Data Figure 4 Diagram of the Ca ii line depths.

Plot of the Ca ii K line depth, pK, as a function of the Ca ii H line depth, pH, for the 252 independent absorption features with α < 1 caused by individual transiting comets observed between 2003 and 2011. Using k-mean cluster analysis of these line depth measurements, two populations of exocomets show up: the 147 exocomets of population S generates the shallow absorption lines with pK < 0.4 (in red) and the 105 exocomets of population D generates the deep absorption lines with pK > 0.4 (in blue). The dotted line represents the full saturation limit pK = pH and the dashed line represents the α = 1 limit, corresponding to cometary cloud with a projected area greater than the stellar disk area.

Extended Data Figure 5 Histogram of the evaporation efficiency of transiting exocomets.

The histogram of η, the evaporation efficiencies (in black), shows a clear bimodal distribution: population S (in red) is centred on ηS = 8.6 ± 0.4, while population D (in blue) is centred on ηD = 9.4 ± 0.1. The solid black histogram represents the distribution of evaporation efficiency for the 252 observed exocomets with α < 1. The two Gaussian curves are obtained by fitting this histogram with the sum of two Gaussians.

Extended Data Figure 6 Histogram of the distances between β Pic and the exocomets at the time of transit.

The 105 comets of population D (in blue) are located further away from the star than the 147 comets of population S (in red), with and . Distances are expressed in units of stellar radius (). The solid black line represents the distribution of distances for the whole sample of observed exocomets.

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Kiefer, F., des Etangs, A., Boissier, J. et al. Two families of exocomets in the β Pictoris system. Nature 514, 462–464 (2014).

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